It is quite easy to check for fuel pressure at the rail using the schrader valve, If you depress the valve, you should see a good stream of fuel shot out. If the pump is not working, you will get a dribble or no fuel escape.

i would guess if you have fuel pump pressure, then go with the crank position sensor.

 

I can't believe my luck....for the first time I have a no-start condition IN the driveway! Yey!

I was able to quickly determine that it is fuel that is being cut.

I have no pressure, and if I'm running to the back of the truck fast enough after turning the key, then it looks like no power to the top of the pump, or to the harness at drivers rear wheel well.

Not sure what to hit up next.

I tried for power on the harness that goes to the inertia switch and got nothing, but i'm not sure if that SHOULD be hot with ignition on, or if that connection times out after 5-seconds like the pump?

 

another update...

I just hooked up a spare battery to the fuel pump and let it run....instantly had about 36psi of fuel pressure.....BUT still just turns over,..no fire.

 

Noticed that after turning the key, there is a ~3 second delay before the starter kicks in. Read something about the 10AS alarm unit in the passenger footwell....took a look and removed it from its bracket to make sure harness plugs were inserted tightly and the hazard 4-way flashers start going off on the truck. Uh-oh.....perhaps this issue is some horrible internal immobilizer problem that is randomly cutting spark and fuel. Hmmnnn??

 

Sounds like what I just went through. I ended up finding the plug for the inertia switch was burnt on one side, I used a large jumper wire that fit real tight and the pump fired right up.

From the inertia switch back to the pump, there is nothing except the connector.

 

Jim,..the inertia switch on the firewall in the engine bay? I already jumped that...still no fuel pump when I did. I also ran 12v to run the pump, which gave me good fuel pressure at the rail, but still no start. Being that this truck runs great some of the time, I think an electrical gremlin the immobilizer system is starting to look likely.

As I understand, the 10AS alarm unit can't just be replaced. It has to be programmed to the ECU.

 

Also noticed the check engine light no longer illuminates when the ignition is switched on. Found this official TSB that seems to match my symptoms... GEMS no-start TSB

 

Yep...pulled out the ECU and alarm unit. As you can see the bottom of the ECU case had turned to dust. Connected up a good used ECU/Alarm unit pair, turned the key and....nothing.

Wait...perhaps the pair need to be 'unlocked'.

Get out, and lock, then unlock the doors. Turn the key....fires right up.

 

Great discovery on the Discovery!
Thanks for sharing!